Rapid Screening of Calcium Carbonate Precipitation in the Presence of Amino Acids: Kinetics, Structure, and Composition

Abstract

Soluble additives are widely used to control crystallization, leading to definition of properties including size, morphology, polymorph and composition. However, due to the number of potential variables in these experiments, it is typically extremely difficult to identify reaction conditions -as defined by solution compositions, temperatures and combinations of additives -that give the desired product. This article introduces a high throughput methodology which addresses this challenge and enables the streamlined preparation and characterization of crystalline materials. Using calcium carbonate precipitated in the presence of selected amino acids as a model system, we use well plates as micro volume crystallizers, and an accurate liquid handling pipetting workstation for sample preparation. Following changes in the solution turbidity using a plate reader delivers information about the reaction kinetics, while semi automated scanning electron microscopy, powder XRD and Raman microscopy provide structural information about the library of crystalline products. Of particular interest for the CaCO3 system is the development of fluorescence based protocols which rapidly evaluate the amounts of the additives occluded within the crystals. Together, these methods provide a strategy for efficiently screening a broad reaction space, where this can both accelerate the ability to generate crystalline materials with target properties, and develop our understanding of additive directed crystallization.

Water: How Does It Influence the CaCO3 Formation?

“…[136] In particular, phosphates, [66] sulfates, [136] and carboxy-containing molecules including citrate, [161] phytic acid, [164] and amino acids such as glycine, [162] aspartic acid, [136,162] phosphoserine, [165] and phosphothreonine [165] have been reported to strongly increase the lifetime of ACCp articles.T he increased kinetic stability of these particles must be related to ad elayed dissolution of ACCo r aretarded formation of CaCO 3 crystals.The influence of lowmolecular-weight additives on the dissolution rate of ACC particles remains unclear. [170] Certain low-molecular-weight additives also delay the crystallization of ACCp articles that are not in contact with bulk water and hence undergo as olid-state transformation. Indeed, the kinetic stability of ACCagainst crystallization in the presence of bulk water increases with the increasing ability of these additives to inhibit crystal growth.…”

“…[172] Thep resence of certain additives that possess ad efined chirality,i ncluding aspartic acid or glutamic acid, can even result in the formation of CaCO 3 crystals with chiral morphologies. [170] C) TGA (solid line) and DSC (dashed line) analysis of ACC particles synthesized in the presence of different amounts of citrate (CIT). [173] Moreover, these low-molecular-weight additives can influence the mechanical properties of the resulting crystals:When7mol %glycine or 4mol %a spartic acid is incorporated into calcite single crystals,t heir hardness increases twofold, as shown in Figure 9F.…”

“…[136] B) Influence of the amino acid/Ca 2+ molar ratio on the lifetime of ACC particles dispersed in an aqueous solutions, measured as the time at which the maximum absorbance appears in turbidimetricanalysis, for aspartic acid (black), glutamic acid (red), asparagine (blue), and valine (pink). [123] Reproducedw ith permission from John Wiley & Sons, [136,161] the American ChemicalS ociety, [170] and Springer Nature. The critical crystallization temperatures are marked with arrows.…”

Wasser: Wie beeinflusst es die CaCO3‐Bildung?

“…As a result of this correlation, the stability of ACC particles sequentially increases if functionalized with valine, asparagine, glutamic acid, and aspartic acid due to their increasing affinity towards the CaCO3 crystal surface, as shown in Figure 10B. [175] Certain low molecular weight additives also delay the crystallization of ACC particles that are not in contact with bulk water and hence undergo a solid-state transformation. These additives do not significantly change the structure or overall degree of hydration of ACC.…”

“…[135] (B) Influence of the molar ratio of amino acids:Ca 2+ on the lifetime of ACC particles dispersed in an aqueous solutions, measured as the time where the maximum absorbance appears in turbidimetric analysis, for aspartic acid (black), glutamic acid (red), asparagine (blue), and valine (pink). [175] (C) TGA (solid line) and DSC (dashed line) analysis of ACC particles synthesized in the presence of different amounts of citrate (CIT). The critical crystallization temperatures are marked with arrows.…”